An electronic packaging process typically comprises various process steps that are operative to attach an integrated-circuit (“IC”) chip or die to a carrier, such as a semiconductor substrate, and then forming a protective encapsulation for the resulting semiconductor device. During attachment of the IC chip, besides mechanical connections, it is important to establish electrical routes for connections between the chip and the substrate.
A chip attachment method that is becoming increasingly popular involves the use of ultrasonic bonding energy in a process referred to as ultrasonic flip chip bonding. Flip chip bonding involves a process that mechanically and electrically connects a chip to a substrate through a series of metallic bumps that are formed on the Input/Output sites on either the chip side or the substrate side. The chip or substrate side having no bumps is plated with metallic pads for facilitating metallurgical bonding of the bumps on the pads. This bonding method is implemented by contacting a tip of a bonding tool coupled to an ultrasonic transducer with a chip to be bonded. The ultrasonic transducer generates ultrasonic energy through the tip of the bonding tool, which drives the chip to scrub on the substrate to weld the bumps with the corresponding pads. As a result, the chip is mechanically stuck to the substrate with the bumps as a joint, with the bumps also acting as electrical conduits between the die and the substrate.
U.S. Pat. No. 5,427,301 entitled “Ultrasonic Flip Chip Process and Apparatus” describes one type of ultrasonic flip chip bonding. The flip chip has an active face provided with conductive bumps so that its active face is oriented toward a substrate. The flip chip is placed on the substrate with a pickup arm so that the bumps align with a bonding pattern on the substrate. An ultrasonic horn with a flat distal end is then lowered onto a back side of the flip chip and applies ultrasonic energy to create a diffusion bond between the chip and the substrate. It uses a pickup arm to transfer the chip onto the substrate and a separate ultrasonic horn to conduct ultrasonic bonding. A disadvantage of having a separate pickup arm and ultrasonic horn is that process cycle time is increased by having to move both the devices into position at each bonding location in order to bond each chip. Therefore, it is desirable to integrate the pickup tool with the ultrasonic horn to reduce the need to move multiple devices. In practice, this can be done by including a collet at the distal end of an ultrasonic horn to facilitate holding of the chip and providing means, such as vacuum suction means, to hold the flip chip securely.
FIG. 1 is a side view of a prior art bonding tool having a collet 10 that is holding a chip 12 against a substrate 14 while performing ultrasonic flip chip bonding. The chip 12 is mechanically attached to the substrate 14 through bumps 16, usually formed on the surface of the chip 12. The bumps 16 also electrically connect input/output pads 18 on the respective facing surfaces of the chip 12 and substrate 14. A working surface 20 at the tip of the collet 10 contacts the top surface of the chip 12 and applies a force to the chip 12 during bonding. In order to hold the chip 12 securely, the collet 10 also has an air passage 22 along its central longitudinal axis leading to a vacuum suction source (not shown) to apply a vacuum suction force on the chip 12. Ultrasonic energy is applied from an ultrasonic transducer (not shown) through the working surface 20 of the collet 10 onto the chip 12.
Since the collet has to hold the flip chip securely during bonding, the material used for the collet is crucial for successful bonding of the chip. A basic requirement for the material is that it should be of a high wear resistance to achieve an acceptable lifespan while producing enough friction against the top face of the chip so that the chip can be held by the collet firmly during the said ultrasonic agitation. Moreover, the material should offer high stiffness and minimal damping to the ultrasonic wave so that energy loss at the working surface or the collet itself is minimized. These various requirements pose constraints for the selection of the materials suitable for the bonding tool. A widely adopted material is cemented tungsten carbide, that is, tungsten carbide grains (hard phase) bonded together with cobalt or nickel (soft phase or binder phase), fabricated using powder metallurgy technology.
It is found that the aforesaid WC—Ni (tungsten carbide-nickel) or WC—Co (tungsten carbide-cobalt) did not work well for all bonding situations. A commonly-encountered problem is that the bonding tool cannot hold the chip tightly enough. As a result, the chip may not move harmoniously with the collet when it is driven by the ultrasonic transducer or may move totally out of the phase with the collet. Therefore, a bonding tool comprising an alternative material for making the collet of the bonding tool would be desirable to help to overcome these shortcomings.